There is presently no suitable procedure for carrying out a thrombin assay in plasma. The development of an assay for this purpose based on radio-labelled hirudin is one of the primary objectives of this proposal. The identification of a hirudin source is of paramount importance in this endeavor. While searching for hirudin in extracts of Gnathobdellidae leeches, screening for inhibitors of other clotting enzymes, such as activated Christmas factor (IXa) and activated Stuart factor (Xa) will be carried out. Purification and characterization of a clotting inhbitor detected in extracts from Macrobdella decora will be investigated.